This invention relates to methods and apparatus for performing in vivo blood thermodilution procedures, and more particularly to improved catheter methods and apparatus for use in such procedures.
One standard method for determining cardiac output is the so-called thermodilution method discussed, for example. in U.S. Pat. Nos. 3,651,318; 4,217,910; and 4,236,527. As conventionally employed, this method involves either injecting a bolus of liquid into the bloodstream at a temperature which is cooler or warmer (usually cooler) than blood temperature, or heating a segment of the blood indirectly with electrical resistance heaters, and monitoring the temperature deviation of the blood as a function of time at a location downstream from the location at which the temperature deviation is caused. The area under the resulting temperature deviation vs. time curve (known as the thermodilution curve) is a measure of the rate at which the heart is pumping blood (usually expressed in liters per minute). If cardiac output is high, the area under the thermodilution curve will be relatively small in accordance with the well-known Stewart-Hamilton relationship. Conversely, if cardiac output is low, the area under the thermodilution curve will be relatively large.
Thermodilution procedures involving the introduction of an auxialiary liquid into the blood have a number of possible deficiencies. The accuracy of these procedures depends in part on accurate knowledge of the temperature, volume, and rate of injection of the auxiliary liquid. Liquid volume measurements are difficult to make with extreme accuracy. For example, a syringe may be used, with the result that the volume may only be known to within several percent of the actual volume. Operator error associated with volume measurement and rate of injection may also be a problem. Because the catheters employed may be quite long (approximately 30 inches), it is difficult to know precisely the temperature of the liquid injectate at the point at which it enters the bloodstream near the distal end of the catheter. Heat exchange with the liquid dispensing device (e.g., the syringe), with the catheter, and with the blood and tissue surrounding the catheter upstream of the point at which the liquid is actually released into the blood may mean that the injectate temperature is also known only to within about 5% of the actual temperature. Because of the effects of the auxiliary liquid on the composition of blood stream liquid, auxiliary liquid injection techniques may not be usable repeatedly or at frequent intervals on a given patient for an extended period of time (e.g., three to six times per hour over a period of 24 to 72 hours).
Dye dilution and conductivity dilution techniques, also involving injection of an auxiliary liquid (i.e., a dye or a saline solution) into the bloodstream, are also known (see, for example, U.S. Pat. Nos. 3,269,386; 3,304,413; and 3,433,935). The resulting dye dilution or conductivity dilution curve is similar to a thermodilution curve. Dye dilution and conductivity dilution have some of the same deficiencies as auxiliary liquid thermodilution, namely, difficulty of precisely controlling the rate of injection and measuring the injectate volume, and unsuitability for frequent or repeated use over long periods of time.
The thermodilution techniques involving the use of electrical resistance heaters to heat the blood (see for example, U.S. Pat. Nos. 3,359,974; 4,217,910; and 4,240,441) may have the disadvantage of locally overheating some blood, especially if the heater or a portion of the heater happens to be close to the surrounding blood vessel so that a large amount of heat is applied to a relatively small amount of blood. This can cause undesirable coagulation of the over-heated blood. In addition, thermodilution techniques employing electrical resistance heaters transfer heat from the surface of the heater to the blood by conduction of heat into that "film" of blood in direct contact with the heated surface. Consequently, only a confined thickness of blood can be heated, limited by conduction heat transfer in blood. The amount of thermal energy that can be transferred to the blood is therefore limited since studies have shown that the blood cannot be heated to temperatures greater than 5.degree. C. above the blood temperature without incurring blood cell damage and the formation of fibrin deposits on the surface of the heating element. Such deposits impede the transfer of heat from the surface of the heating element to the blood. The consequence of limiting the amount of heat which can be delivered to the blood by prior art methods is that the amount of thermal energy imparted to the blood, i.e., the "indicator", is too small to allow for the accurate measurement of cardiac output by conventional thermodilution techniques.
In view of the foregoing, it is an object of this invention to improve the methods and apparatus employed in thermodilution procedures.
It is more particular object of this invention to provide methods and apparatus for improving the reliability, accuracy, and repeatability of thermodilution procedures.
It is still another object of the invention to provide thermodilution methods and apparatus that can be used on a given patient with increased frequency over time periods of increased length.